The distance traveled is immense: The disease is now endemic in only three countries around the globe—Afghanistan, Nigeria, and Pakistan—and its incidence has decreased by more than 99 percent from an estimated 350,000 cases in 1988. (Endemic is the term used to describe countries with an ongoing incidence of the disease.)

But we're not quite there yet, as this year's spring outbreaks in previously polio-free Somalia and Kenya reminded us. In addition, the discovery of sewage samples containing poliovirus in Israel has led to a countrywide campaign to offer oral polio vaccines to children between the ages of four months and nine years, as a precaution.

No cases have appeared in Israel. And the numbers of those affected in Somalia and Kenya are small—110 as of August 7—with 177 reported cases worldwide so far this year, according to data from the Global Polio Eradication Initiative, an international group spearheaded by the World Health Organization (WHO). In April the group issued a detailed strategic plan to eliminate polio "for all time" by 2018. Fully funded, the comprehensive vaccination and monitoring and surveillance plan would cost about $5.5 billion.

But in the meantime, this highly infectious nerve disease persists, potentially causing lifelong paralysis in the young children it most often targets.

Why has the end remained elusive, and what is being done to protect against future setbacks?

Polio in Perspective

It's important to remember there is no cure for polio; it exists only in humans, and it can only be prevented by vaccination, points out Dr. Jay Wenger, director of the polio eradication program at the Bill and Melinda Gates Foundation. Put those factors together and it means that "to eradicate polio we essentially have to vaccinate enough children so that the poliovirus has no place to go," he says.

In the United States, effective vaccine campaigns have kept the population polio free since 1979. But as recently as the late 1940s and early 1950s—before the anti-polio vaccines were developed—the disease disabled approximately 35,000 people, many of them children, here each year.

By contrast, between 1988 and today, the number of polio-endemic countries has gone from 125 to just 3, and in 2012 only 223 cases were recorded worldwide.

Given that larger picture, "it's important to put the issue of setbacks into the context of the program and recognize that progress continues," says Dr. Hamid Jafari, director of the WHO's Global Polio Eradication Initiative. Or, as Dr. Wenger puts it: "These outbreaks highlight the importance of eliminating the virus" in the remaining endemic countries.

Logistics, Logistics, Logistics

Vaccines are the answer. The challenge lies in identifying, locating, and then reaching people of all ages who have not yet been vaccinated. For example, highly mobile groups, such as Nigeria's nomadic livestock herders, may spread the disease as they travel from place to place. (According to the WHO, the polio strains affecting residents of the Horn of Africa originated in West Africa.)

At the same time, armed conflict, political unrest, and what Dr. Jafari calls "complex geopolitical situations" can make access by vaccine workers at times dangerous or difficult, if not impossible. Such conditions are present in parts of all three countries where polio still exists, making different pockets or areas insecure for efficient vaccine delivery at different times. Public health infrastructure can also be spotty, if it exists at all.

Because each area can present a distinct set of obstacles, detailed and tailored strategies need to be worked out for each situation, including partnerships with local authorities, community groups, traditional leaders, and non-governmental organizations (NGOs), says Dr. Jafari. "Fundamental commitment to the program from the top of the government down to the local authorities" is needed if everyone is to be vaccinated.

Resistance, Distrust, and Violence

Mistrust of outsiders is another major obstacle, brutally dramatized by the targeted killings of polio vaccine workers in Nigeria and Pakistan over the past year. In Pakistan's North and South Warziristan, the Taliban have banned vaccination since June 2012, leaving children without immunity and at high risk for the disease.

Dr. Jafari is stoic, but not despairing. "As long as there are authorities on the ground and mothers who want to protect their children," there is a way to make progress, he says. "That is why, despite the shootings, the people come out with their children to get them vaccinated ... and these brave men and women are going out in their communities to vaccinate" the children.

Environmental Vigilance

Beyond vaccination, surveillance and monitoring the presence of the virus in the environment are essential to eradicating polio. It's not known how the virus was carried to Israel, which detected poliovirus in sewage samples starting in February 2013. According to the WHO, the strain in Israel is related to the South Asian cluster of wild poliovirus currently circulating in Pakistan, which had recently also been detected through environmental sampling in Egypt.

The current oral vaccine campaign in Israel—which is polio free and has a very high rate of immunization—is a safeguard, a direct response to the detection of poliovirus in its sewage. The goal of Israel's vaccine campaign is to boost immunity levels even in children who have already received the injected vaccine, as well as to protect against further spread to anyone traveling in or out of Israel.

Setbacks on the Radar

Against the backdrop of all these issues, the setbacks in Kenya and Somalia show how quickly and far poliovirus can travel, and demonstrate the importance of maintaining high immunity levels within populations and the need for strong surveillance to halt its spread.

According to Dr. Jafari, such setbacks will happen "as long as the virus is alive and people are moving with the virus, and it will spread as people move. And when it lands in places where immunization and sanitation are not in place, that is where the setbacks will take place."

The key is being prepared to deal with them, while continuing to move forward. "We're on track, and we'll deal with these outbreaks. They are not obstacles; they are part of the course."